The invention relates to thermomechanical treatment of pieces comprised of high grade microalloy steel, which treatment may consist of, e.g., forging, welding, or surface treatment.
In particular, steels of the following composition are used in producing high performance forgings:
______________________________________ C 0.05-0.5 wt. % Mn 1-2 wt. % Si 0.05-1.5 wt. % Cr 0.1-1 wt. % Mo 0-0.5 wt. % V 0-0.30 wt. % B 0-0.010 wt. % Ti 0-0.030 wt. % Nb 0-0.1 wt. % Fe (major component) ______________________________________
Such steels may also include other alloying elements compatible with the intended uses, and various accompanying impurities not removed in processing. The resulting steel pieces may have a tensile strength of between 900-1200 MPa, e.g., and good resistance to transients (shock resistance and impact resistance). Such steels are classically used with a ferrito-perlitic structure which results naturally from secondary hardening during cooling in connection with the final forming operation. Such steels having a ferrito-perlitic structure can also be used for other applications requiring various thermal or thermomechanical treatments.
This type of metallurgical processing is well suited for forging and other operations carried out at elevated temperatures (greater than 1000.degree. C., for example), for which the austenitization of the piece is complete. However, if part or all of the piece is subjected to lower heating temperatures in the range of 500.degree.-900.degree. C., problems can occur. Such a problematic situation may occur when the forging is only carried out on one end of a steel bar, and the heating (above 1000.degree. C.) is confined to said end. The region of the bar which is not directly heated but which neighbors the zone to be forged becomes heated to a temperature which is lower but which is still sufficiently high to cause metallurgical changes in this region. A similar phenomenon occurs when the piece is subjected to welding, nitriding, or a thermal surface treatment such as laser tempering, or induction tempering--in such cases, the subsurface regions of the piece may be affected by the treatment in a manner which is very undesirable. Specifically, the parts of the piece subjected to such heating between 500.degree. and 900.degree. C. may be softened in a way which is detrimental to the strength of the piece, particularly its fatigue strength. One explanation of this softening phenomenon is that for the relatively low heating temperatures (500.degree.-900.degree. C.) the hardening effect of the interphase precipitations is reversed, and temperability is substantially reduced because of the very small grain size. As a result, the piece no longer has the desired properties, and in particular does not have them homogeneously throughout its entire volume.
Heretofore, attempts have been made to remedy this problem by employing chromium-molybdenum steels with tempering and annealing, or low alloy steels with a normalization treatment. However, these solutions limit the range of alloys and grades which can be employed. Further, where a thermal treatment to restore the mechanical properties of the piece is needed, it is generally accompanied by unacceptable deformations which necessitate some sort of correction, introducing appreciable additional time and costs into the manufacturing process for the piece.